This proposal seeks to characterize the cellular mechanisms underlying neuronal development and plasticity in Drosophila. It investigates the roles of second messenger systems, membrane excitability, membrane cycling, and cell adhesion, using methods of genetics, physiology, and cell biology. The proposal addresses a wide variety of issues relating to neuronal development and plasticity. It contains 9 specific aims, and the principal issues addressed are: 1) the role of second messenger systems in organizing the neuronal cytoskeleton and in the physiology of the neuromuscular junction; 2) the influence of nerve excitability and activity on cytoskeletal organization in cultured neurons and flight muscles; 3) the role of membrane vesicle proteins in regulating the addition of new membranes in neuronal development (e.g. as reflected in growth cone formation and neurite growth rates); 4) the role of the cytoplasmic tyrosine kinase abl and related proteins in cytoskeletal organization and neuronal adhesion.